Mechanisms of the odd-even effect and its reversal in rectifying performance of ferrocenyl-n-alkanethiolate molecular diodes

Using nonequilibrium Green's function method combined with density functional theory, here we show that the rectification performance of molecular diodes, consisting of a single n-alkanethiolate molecule with a ferrocenyl head group sandwiched between two metal (Au or Ag) electrodes, can be modulated by tuning the number n of methylene units in the ferrocenyl-n-alkanethiolate backbone. Specifically, there is an odd-even effect in the rectifying performance as the methylene unit number n in ferrocenyl-n-alkanethiolates is varied. More interestingly, a reversal of the odd-even effect is observed when the Au electrodes are replaced by Ag electrodes. Further analysis reveals that the rectification is governed by the monotonic evolution of the strongly localized frontier molecular orbitals under external bias voltages. Meanwhile, the odd-even effect in the rectification ratios is attributed to different alignments between frontier molecular orbitals and Fermi energy of electrodes for odd- and even-numbered ferrocenyl-n-alkanethiolates, which is originated from the odd-even dependent coupling strength between the ferrocenyl head group and the adjacent electrode. And the odd-even dependence of coupling strength is completely reversed on Au and Ag electrodes due to different Au-S-C and Ag-S-C bond angles that contributes to the reversal of the odd-even effect in rectification ratios. This work is helpful for future rational design and performance control of molecular diodes. (C) 2017 Elsevier B.V. All rights reserved.